1. Field of the Invention
The present invention is directed to a method for rapidly forming polysilicon from amorphous silicon, and more particularly, to using lasers to form crystallized polysilicon channels for thin film transistors.
2. Discussion of the Prior Art
Thin film transistors (TFTs) are critical for high performance liquid crystal display (LCD), which are one of the most important components for laptop computers. TFTs are also applied to other two dimensional (2D) imagers, sensors, and electronic equipment. Currently, most large area arrays of TFTs are based on amorphous materials, such as a hydrogen amorphous silicon a-Si:H. However, a-Si:H TFT has some intrinsic drawbacks, e.g., low mobility and high photosensitivity. Therefore, several extra process steps have to be added to the manufacturing procedure to avoid, or compensate for, these problems. For example, a black matrix has to be used to block light from reaching the TFTs. Drivers for a display have to be fabricated separately from the TFTs of the array.
To avoid the problems associated with a-Si:H based TFTs, polysilicon TFTs are used. One of the major drawbacks of a polysilicon TFT is high leakage current. Proper design of the polysilicon TFT structure minimizes the leakage current. Display panel fabrication procedure is simplified and cost is reduced where all the driver circuits are integrated into the pixel TFT fabrication process. However, a major problem in fabricating polysilicon TFTs is the formation of polysilicon under certain conditions which include:
1) a low temperature, such as less than 550.degree. C. on a low temperature glass; PA1 2) on a large piece of glass substrate; and PA1 3) with a high throughput. PA1 1. A. Kohno, T. Sameshima, N. Sano, M. Sekiya, and M. Hara, IEEE Trans. Electron Devices 42 (2), 251 (1995); and PA1 2. H. Tanabe, K. Sera, K, Nakamura, K. Hirata, K. Yuda and F. Okumura, NEC Res. & Dev. 35 (3), 254 (1994). PA1 3. K. Ono, S. Oikawa, N. Konishi, and K. Miyata, JPN. J. Appl. Phys. 29, 2705 (1990). PA1 4. M. Bonnel, N. Duhamel, M. Guendouz, L. Haji, B. Loisel, and P,. Ruault, Jpn. N. Appl. Phys. 30 (1B), L 1924 (1991); PA1 5. G. Liu and S. J. Fonash, Appl. Phys. Lett. 62, 22554 (1993); and PA1 6. S. W. Lee, Y. C. Jeon, and S. K. Joo, ECS Proceedings of 2nd Thin Film Transistor Technologies, edited by Y. Kuo. (Electrochemical Society, Pennington, N.J. (1994), Vol. 94-35, p. 115).
Therefore, high temperature processes, such as annealing at 700.degree. C., are not suitable for low temperature glass. Several methods, including laser crystallization, furnace annealing and reactive chemical vapor deposition have been reported for the preparation of polysilicon. These methods require either high temperature or a long process time. In some cases, uniformity over a large area is difficult to achieve. Therefore, using conventional methods, high quality polysilicon cannot be formed in an efficient manner.
The use of laser annealing to supply polysilicon with low defect densities within grains is disclosed in the following references:
Laser polysiliconization is popular because it produces high mobility TFTs. However, laser annealing has drawbacks. One approach would be to scan the entire amorphous silicon layer with a laser beam since the laser beams used for this purpose are usually small, e.g., less than 1 cm by 1 cm, and it takes a long time to scan through the whole area of a large display substrate. Further, even if a large field laser is used, the crystallization process is tedious. The laser beam spot is not uniform across its diameter. To compensate for this nonuniformity, complicated beam overlap procedures have to be used to obtain uniformity of the polysilicon structure. Another approach would be to selectively crystalize only those areas of the amorphous silicon film to be occupied by the TFTs. However, later alignment of the TFT structure on the polysilicon areas would be critical.
Another polysilicon crystallization method is to use a low-temperature, e.g., 600.degree. C., furnace annealing to crystallize silicon. Such a method is discussed in the following reference, which is incorporated herein by reference:
There are reports on direct deposition of polycrystalline or microcrystalline silicon films at temperatures lower than 500.degree. C. by adding hydrogen-, fluorine-, or chlorine-containing gases to the silicon source in a chemical vapor deposition (CVD) process. Most of the films formed by such a reactive CVD process have a columnar structure with rough topography. It is difficult to obtain a good polysilicon film uniformity over a large substrate area with this kind of reactive CVD. Thus, highly reactive CVD processes have not provided good quality polysilicon film over a large area.
The following references discuss polysilicon formation processes:
When a proper metal is in contact with the original amorphous silicon, both the temperature and the time of the crystallization can be shortened. For example, the Bonnel, et al. reference showed that polysilicon could be formed in 40 seconds at 750.degree. C. when the amorphous silicon was in contact with indium tin oxide (ITO). The Liu et al. reference reported that when a thin layer, i.e., 40 .ANG., of palladium (Pd) was deposited underneath an amorphous silicon, the crystallization could be carried out in 2 hours at 600.degree. C. The Lee et al. reference demonstrated that silicon could be crystallized laterally from the Pd contact area. Although the growth temperature was low, i.e., 500.degree. C., it took 10 hours to fully crystallize an area of 100 micrometers by 100 micrometers.
These conventional processes are not practical for the mass production of TFTs on a low temperature glass, such as Corning 7059, because either the temperature is too high or the process time is too long.
In the above mentioned U.S. patent application, Ser. No. 08/685,728, filed Jul. 24, 1996 (IBM Docket #Y09-96-092), by the inventor of the present application, rapid silicon crystallization and rapid transformation of amorphous silicon to polysilicon is accomplished by a pulsed rapid thermal annealing (PRTA) method performed on partially fabricated TFTs having a layered metal-silicon structure, where the metal of the structure acts as a seed layer to transform the amorphous silicon to polysilicon. The PRTA method subjects the TFT to relatively high temperature pulses of short duration to obtain silicide at the metal silicon interface. As the high temperature pulses continue, there is a rapid transformation of the amorphous silicon to polysilicon that spreads out from the silicide without causing irreparable damage to low temperature glass of the TFT substrate.
While this technique has proven to be effective, it simultaneously subjects the whole substrate to the high temperature pulses which requires the process to be controlled to provide acceptable results both at the center and around the periphery of the substrate containing the TFTs.